This invention relates to variable-frequency oscillator circuits, and more particularly to digitally controlled oscillator (DCO) circuits.
Voltage controlled oscillator (VCO) circuits are widely used in frequency synthesizers, clock and data recovery (CDR) circuits, and so on. The output frequency of a VCO is tuned by its analog input voltage, and the main parameters for a VCO are center frequency, frequency tuning range, VCO gain, and phase noise. Due to the nature of the frequency tuning, any noise in the control signal will modulate the VCO, resulting in more undesirable noise output. This effect can be reduced by reducing the VCO gain, but the frequency tuning range will be reduced as well because of the limited range of tuning voltage. By employing both coarse tuning and fine tuning, the gain of the fine tuning can be reduced while a large tuning range relies on the coarse tuning (which still has a very large gain).
Recently there has been increasing interest in digitally controlled oscillators (DCOs). DCOs normally have small analog gain, but the frequency resolution is typically limited. In Staszewski et al., “A First Multigigahertz Digitally Controlled Oscillator for Wireless Applications,” IEEE Transactions on Microwave Theory and Techniques, Vol. 51, No. 11, November 2003, pp. 2154-64, a sigma-delta modulator is used to enhance the frequency resolution. However, this results in a complex digital circuit.